The present invention relates generally to implantable pulse generators, e.g., a pulse generator used within a Spinal Cord Stimulation (SCS) system or other type of neural stimulation system. More particularly, the present invention relates to the use of a rechargeable zero-volt technology lithium-ion battery within such an implantable pulse generator.
Implantable pulse generators (IPG) are devices that generate electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a spinal cord stimulation system. A spinal cord stimulation system is a programmable implantable pulse generating system used to treat chronic pain by providing electrical stimulation pulses from an electrode array placed epidurally near a patient""s spine. SCS systems consist of several components, including implantable and external components, surgical tools, and software. The present invention provides an overview an SCS system and emphasizes the use of a rechargeable zero volt technology battery within such a system, including the charging system used for charging the rechargeable battery.
Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. SCS systems typically include an implantable pulse generator, lead wires, and electrodes connected to the lead wires. The pulse generator delivers electrical pulses to the dorsal column fibers within the spinal cord through the electrodes implanted along the dura of the spinal cord. The attached lead wires exit the spinal cord and are tunneled around the torso of the patient to a subcutaneous pocket where the pulse generator is implanted.
Spinal cord and other stimulation systems are known in the art, however, to applicants"" knowledge, none teach the use of a rechargeable zero-volt technology battery within the implanted portion of the system, with accompanying charging and protection circuitry, as proposed herein. For example, in U.S. Pat. No. 3,646,940, there is disclosed an implantable electronic stimulator that provides timed sequenced electrical impulses to a plurality of electrodes so that only one electrode has a voltage applied to it at any given time. Thus, the electrical stimuli provided by the apparatus taught in the ""940 patent comprise sequential, or non-overlapping, stimuli.
In U.S. Pat. No. 3,724,467, an electrode implant is disclosed for the neural stimulation of the spinal cord. A relatively thin and flexible strip of physiologically inert plastic is provided with a plurality of electrodes formed thereon. The electrodes are connected by leads to a RF receiver, which is also implanted and controlled by an external controller. The implanted RF receiver has no power storage means, and must be coupled to the external controller in order for neural stimulation to occur.
In U.S. Pat. No. 3,822,708, another type of electrical spinal cord stimulating device is shown. The device has five aligned electrodes that are positioned longitudinally on the spinal cord and transversely to the nerves entering the spinal cord. Current pulses applied to the electrodes are said to block sensed intractable pain, while allowing passage of other sensations. The stimulation pulses applied to the electrodes are approximately 250 microseconds in width with a repetition rate of from 5 to 200 pulses per second. A patient-operable switch allows the patient to change which electrodes are activated, i.e., which electrodes receive the current stimulus, so that the area between the activated electrodes on the spinal cord can be adjusted, as required, to better block the pain. Other representative patents that show spinal cord stimulation systems or electrodes include U.S. Pat. Nos. 4,338,945; 4,379,462; 5,121,754; 5,417,719 and 5,501,703.
The dominant SCS products that are presently commercially available attempt to respond to three basic requirements for such systems: (1) providing multiple stimulation electrodes to address variable stimulation parameter requirements and multiple sites of electrical stimulation signal delivery; (2) allowing modest to high stimulation currents for those patients who need it; and (3) incorporating an internal power source with sufficient energy storage capacity to provide several years of reliable service to the patient. Unfortunately, not all of these features are available in any one device. For example, one known device has a limited battery life at only modest current outputs, and has only a single voltage source, and hence only a single stimulation channel (programmable voltage regulated output source), which provides a single fixed pattern to up to four electrode contacts. Another known device offers higher currents that can be delivered to the patient, but does not have a battery, and thus requires the patient to wear an external power source and controller. Even then, such device still has only one voltage source, and hence only a single stimulation channel, for delivery of the current stimulus to multiple electrodes through a multiplexer. Yet a third known device provides multiple channels of modest current capability, but does not have an internal power source, and thus also forces the patient to wear an external power source and controller. It is thus seen that each of the systems, or components, disclosed or described above suffers from one or more shortcomings, e.g., no internal power storage capability, a short operating life, none or limited programming features, large physical size, the need to always wear an external power source and controller, the need to use difficult or unwieldy surgical techniques and/or tools, unreliable connections, and the like. What is clearly needed, therefore, is a spinal cord stimulation system that is superior to existing systems by providing longer life through the use of a rechargeable battery, easier programming and more stimulating features in a smaller package without compromising reliability.
Regardless of the application, all implantable pulse generators are active devices requiring energy for operation, either powered by an implanted battery or an external power source. It is desirable for the implantable pulse generator to operate for extended periods of time with little intervention by the patient or caregiver. However, devices powered by primary (non-rechargeable) batteries have a finite lifetime before the device must be surgically removed and replaced. Frequent surgical replacement is not an acceptable alternative for many patients. If a battery is used as the energy source, it must have a large enough storage capacity to operate the device for a reasonable length of time. For low-power devices (less than 100 xcexcW) such as cardiac pacemakers, a primary battery may operate for a reasonable length of time, often up to ten years. However, in many neural stimulation applications such as SCS, the power requirements are considerably greater due to higher stimulation rates, pulse widths, or stimulation thresholds. Powering these devices with conventional primary batteries would require considerably larger capacity batteries to operate them for a reasonable length of time, resulting in devices so large that they may be difficult to implant or, at the very least, reduce patient comfort. Therefore, in order to maintain a device size that is conducive to implantation, improved primary batteries with significantly higher energy densities are needed. However, given the state of the art in battery technology, the required energy density is not achievable at the present time.
If an implanted battery is not used as the power source, then a method is required to transcutaneously supply power to the IPG on a continuous basis. For applications that require large amounts of power such as heart pumps and other heart-assist devices, an external power source is the preferred choice. Power can be supplied to the device via a percutaneous cable, or more preferably and less invasively, coupled to the device through electromagnetic induction. The external power source can be an AC outlet or a DC battery pack, which may be recharged or replaced with new batteries when depleted. However, these systems obviously require the patient to continually wear an external device to power the implanted pulse generator, which may be unacceptable for many patients because they are often bulky and uncomfortable to wear, and naturally, limit patient mobility.
One alternative power source is the secondary, or rechargeable battery, where the energy in these batteries can be replenished by recharging the batteries on a periodic basis. It is known in the art to use a rechargeable battery within an implant device. See, e.g., U.S. Pat. No. 4,082,097, entitled xe2x80x9cMultimode Recharging System for Living Tissue Stimulatorsxe2x80x9d, and applicant Mann""s U.S. patent application Ser. No. 09/048,826, filed Mar. 25, 1998, entitled xe2x80x9cSystem of Implantable Devices for Monitoring and/or Affecting Body Parametersxe2x80x9d, which patent and patent application are likewise incorporated herein by reference. The devices and methods taught in this patent and application, however, comprise specialized devices, e.g., microstimulators, or relate to specific applications, e.g., cardiac pacing, which impose unique requirements not applicable to many IPG applications. Cardiac pacemakers with rechargeable batteries have been developed in the past; see U.S. Pat. Nos. 3,454,012; 3,824,129; 3,867,950; 3,888,260; and 4,014,346. However, these devices were met with limited success in regards to battery performance and market acceptance. Many of these devices were powered by nickel-cadmium (NiCd) batteries. NiCd""s low volumetric energy density of 100 Wh/liter provided limited energy storage, and frequent charging was required. Also, its low nominal cell voltage of 1.2 V required many cells to be stacked in series, requiring cells to be closely matched for optimum performance. NiCd batteries also suffered from a phenomenon called xe2x80x9cmemory effect,xe2x80x9d which causes the cell to lose capacity if cycled at shallow discharge depths. Moreover, NiCd batteries have a high self-discharge rate, losing approximately 30% of their capacity per month at body temperatures. Also, cycle life performance was poor, as NiCd batteries typically lasted fewer than 300 cycles. In addition, charging NiCd batteries was often problematic because the standard charge termination method for NiCd batteries is somewhat complicated, requiring the need to detect a zero or negative voltage slope (dV/dt) and/or temperature slope (dT/dt). When NiCd batteries are overcharged, an exothermic reaction occurs: oxygen gas given off at the nickel electrode recombines with the cadmium electrode to form cadmium hydroxide. Cell leakage or venting can occur as a result of the pressure increase in the cell. Furthermore, there may be disposal issues with NiCd batteries, as cadmium is highly toxic to the environment.
Newer battery technologies have been developed in recent years. The Nickel Metal-Hydride (NiMH) battery was developed to improve upon NiCd performance. NiMH batteries were first commercially introduced in 1990, and are in many ways similar to NiCd batteries. The main exception is the replacement of the cadmium electrode with a metal-hydride alloy, resulting in more than twice the volumetric energy density ( greater than 200 Wh/liter). In addition, the metal-hydride is less toxic than cadmium. However, NiMH batteries suffer from some of the same drawbacks as well, including low cell voltage (1.2 V), high self-discharge ( greater than 30% per month), difficult charge termination, low cycle life ( less than 300 cycles), and to a lesser extent, memory effect.
Rechargeable lithium-based batteries were first developed in the 1970s using lithium metal as the active electrode material. Lithium has great promise as a battery material because it is the lightest of all metals, with high cell voltage ( greater than 3 V) and high energy density. However, lithium metal in its pure form is extremely reactive, and proved to be very unstable as a battery electrode as employed in early designs. In 1990, however, Sony Corporation introduced a safer rechargeable lithium-based battery called lithium-ion (Li-ion), which used a lithium composite oxide (LiCoO2) cathode and a lithium-intercalating graphite anode. Lithium ions, or Li+, instead of lithium metal, are shuttled back and forth between the electrodes (hence the nick-name, xe2x80x9crocking-chairxe2x80x9d battery). Lithium-ion is superior to other rechargeable battery chemistries, with the highest volumetric energy density ( greater than 300 Wh/liter) and gravimetric energy density ( greater than 100 Wh/kg). In addition, Lithium-ion batteries have a high nominal voltage of 3.6 V, as well as low self-discharge (less than 10% per month), long cycle life ( greater than 500), and no memory effect. Charge termination for Lithium-ion batteries is also simpler than that of NiCd and NiMH batteries, requiring only a constant voltage cutoff. However, Lithium-ion batteries are not as tolerant to overcharging and overdischarging. If significantly overcharged, Lithium-ion batteries may go into xe2x80x9cthermal runaway,xe2x80x9d a state in which the voltage is sufficiently high to cause the electrode/electrolyte interface to breakdown and evolve gas, leading to self-sustaining exothermic reactions. As a result, cell leakage or venting can occur. If Lithium-ion batteries are over-discharged ( less than 1 V), the negative electrode may dissolve and cause plating of the electrodes. This can lead to internal shorts within the cell, as well as possible thermal runaway. Therefore, careful monitoring of the cell voltage is paramount, and battery protection circuitry is necessary to keep the cell in a safe operating region.
It is known in the art to use a Lithium-ion battery in an implantable medical device, see. e.g., U.S. Pat. Nos. 5,411,537 and 5,690,693. However, such disclosed use requires careful avoidance of overcharge and overdischarging conditions, as outlined above, else the implant battery, and hence the implant device, is rendered useless.
The most recent development in rechargeable battery technology is the Lithium-ion polymer battery. Lithium-ion polymer batteries promise higher energy density, lower self-discharge and longer cycle life compared to conventional aqueous electrolyte Lithium-ion batteries. Its chemical composition is nearly identical to that of conventional Lithium-ion batteries with the exception of a polymerized electrolyte in place of the aqueous electrolyte. The polymer electrolyte enables the battery to be made lighter and thinner than conventional Lithium-ion batteries by utilizing foil packaging instead of a metal can, thus allowing it to be conformable to many form factors. Lithium-ion polymer batteries are also theoretically safer since the polymer electrolyte behaves more benignly when overcharged, generating less heat and lower internal cell pressure. Unfortunately, commercial realization of Lithium-ion polymer batteries has been slow and fraught with early production problems. Only recently have the major battery manufacturers (Sony, Panasonic, Sanyo) announced plans for Lithium-ion polymer battery production.
What is clearly needed for neural stimulation applications is a physically-small power source that either provides a large energy reservoir so that the device may operate over a sufficiently length of time, or a replenishable power source that still provides sufficient energy storage capacity to allow operation of the device over relatively long period of time, and which then provides a convenient, easy and safe way to refill the energy reservoir, i.e., recharge the battery, so that the device r may again operate over a relatively long period of time before another refilling of the reservoir (recharging of the battery) is required.
A spinal cord stimulation (SCS) system that uses a rechargeable battery has been invented that is superior to existing systems. Because physically-small power sources suitable for implantation having sufficient capacity to power most neural stimulation applications do not yet exist, the power source used in an neural stimulation IPG (or other implantable medical device application) in accordance with the teachings of the present invention is a rechargeable battery. More particularly, the present invention is directed to the use of a rechargeable lithium-ion or lithium-ion polymer battery within an implantable medical device, such as an implantable pulse generator (IPG), coupled with the use of appropriate battery protection and battery charging circuits.
In accordance with one aspect of the invention, therefore, a lithium-ion or lithium-ion polymer rechargeable battery is used in combination with appropriate battery protection and charging circuitry housed within an implantable medical device, e.g., an IPG, of a medical system, e.g., an SCS system. Such use of a rechargeable battery advantageously assures the safe and reliable operation of the system over a long period of time. While a preferred embodiment of the invention is represented and described herein by way of a spinal cord stimulation (SCS) system, it is to be emphasized that the inventionxe2x80x94directed to the use of a lithium-ion or lithium-ion polymer rechargeable battery in an implanted medical device, including appropriate battery protection and battery charging circuitryxe2x80x94may be used within any implantable medical device.
The representative SCS system with which the lithium-ion based rechargeable battery is employed in accordance with the present invention provides multiple channels, each able to produce up to 20 mA of current into a 1 Kxcexa9 load. To provide adequate operating power for such a system, the SCS system employs a rechargeable battery and charging/protection system that allows the patient to operate the device independent of external power sources or controllers. Moreover, the implanted battery is rechargeable using non-invasive means, meaning that the battery can be recharged as needed when depleted by the patient with minimal inconvenience. Advantageously, the SCS system herein described requires only an occasional recharge, is smaller than existing implant systems, has a life of at least 10 years at typical settings, offers a simple connection scheme for detachably connecting a lead system thereto, and is extremely reliable.
A key element of the SCS system herein described (or other system employing an implantable pulse generator, or xe2x80x9cIPGxe2x80x9d) is the use of a rechargeable lithium-ion or lithium-ion polymer battery. Lithium-ion batteries offer several distinct advantages over other battery chemistries: high volumetric and gravimetric energy densities, high cell voltage, long cycle life, simple detection of charge termination, low toxicity, and no memory effect.
The lithium-ion or lithium-ion polymer battery used in the SCS system described herein is specifically designed for implantable medical devices. It incorporates several distinct features compared to conventional lithium-ion batteries. The battery case is made from a high-resistivity Titanium alloy to reduce heating from eddy currents induced from the electromagnetic field produced by inductive charging. The case is also hermetically-sealed to increase cycle life and shelf life performance. Most importantly, the battery is specifically designed to allow discharge to zero volts without suffering irreversible damage, which feature is referred to herein as xe2x80x9czero-volt technologyxe2x80x9d. This feature is significant because conventional lithium-ion batteries can not operate at low voltages (less than about 1 V) without damage occurring to the negative electrode. Thus, should an implant device with a conventional lithium-ion battery be operated until the implanted battery is nearly discharged (xcx9c2.5 V), and if the battery is not subsequently recharged for any one of many reasons, the battery will naturally self-discharge to below 1 V in less than six months. If this occurs, the performance and safety of the cell may be compromised. In contrast, the present invention relates to an implantable electrical stimulator capable of recharging its lithium-ion battery from an electrical potential of 0 V up to normal operating voltages, e.g., approximately 4 V. The invention takes advantage of new battery technology that allows discharge down to 0 V without damage to the cell.
In accordance with one aspect of the present invention, the SCS system utilizes a non-invasive, electromagnetic induction system to couple the energy from an external power source to the implanted charging circuitry for recharging the battery. The charging circuitry contains a charge controller that converts the unregulated induced power into the proper charging current. The level of the charging current is determined by a state machine-type algorithm that monitors the voltage level of the battery. In one embodiment, when the battery voltage is below 1 V, for example, the battery is charged with a very low current of C/50 (1/50 of the battery capacity) or less. When the battery voltage surpasses 1 V, the battery is charged at a rate of approximately C/25. When the battery voltage surpasses 2.5 V, the battery is charged at the maximum charge rate of approximately C/2, until the battery voltage nears its desired full-charge voltage, at which point the charge rate may again be reduced. That is, fast charging occurs at the safer lower battery voltages (e.g., voltage above about 2.5 V), and slower charging occurs when the battery nears full charge higher battery voltages (above about 4.0 V). When potentially less-than-safe very low voltages are encountered (e.g., less than 2.5 V), then very slow (trickle) charging occurs to bring the battery voltage back up to the safer voltage levels where more rapid charging can safely occur. This multi-rate charge algorithm minimizes charging time while ensuring the cell is safely charged. The charging circuitry also contains a battery protection circuit that continuously monitors the battery voltage and current. If the battery operates outside of a predetermined range of voltage or current, the battery protection circuitry disconnects the battery from the particular fault, i.e. charging circuitry or load circuits. Moreover, the charging circuitry is able to monitor the state-of-charge of the battery by measuring the voltage of the battery, since there is good correlation between battery voltage and state of charge in lithium-ion batteries.
In accordance with another aspect of the invention, an external charging system is provided that supplies the energy to the rechargeable battery of the IPG device. Such external charging system may take one of several forms or embodiments. In one embodiment, the external charger is powered by an alternating current (AC) power supply and is manually controlled by the patient. In another embodiment, the external charger is powered by an AC power supply and is automatically controlled by the external controller for the implanted device. Such embodiment necessarily employs a suitable communication link between the external controller and the external charger, the communication link comprising, e.g., a cable (hard wire connection), an infrared (IR) link, or a radio frequency (RF) link. In another preferred embodiment, the external charger is itself powered by a battery, which battery may be a replaceable (primary) battery, or a rechargeable battery.
The external charger may thus assume one of several forms, ranging from a table-top AC powered device to a small portable (mobile) device that uses a primary or secondary battery to transfer energy to the implanted device. In all instances, the electrical circuitry within the implanted device has final control upon the acceptance or rejection of incoming energy. The external charging system, however, is optimally controlled so that its operation is terminated if the implanted device does not require the external energy.
In operation, the SCS system (or other system employing an IPG) monitors the state of charge of the internal battery and controls the charging process. Then, through a suitable communication link, the SCS system is able to inform the patient or clinician regarding the status of the system, including the state of charge, and makes requests to initiate an external charge process. In this manner, the acceptance of energy from the external charger is entirely under the control of the implant circuitry, e.g., the IPG, and several layers of physical and software control may be used, as desired or needed, to ensure reliable and safe operation of the charging process. The use of such a rechargeable power source thus greatly extends the useful life of the SCS system, or other IPG systems. This means that once the IPG is implanted, it can, under normal conditions, operate for many years without having to be explanted.
All of the above and other features combine to provide a SCS system employing an IPG or similar implantable electrical stimulator (or other implantable electrical circuitry, such as an implantable sensor) having a rechargeable battery that is markedly improved over what has heretofore been available.